The present invention relates to a current collector sheet for carrying an electrode material mixture used for an electrochemical device and an electrochemical device.
As electronic and electric equipment is made smaller and lighter, demand is increasing for smaller and lighter electrochemical devices, particularly for smaller and lighter secondary batteries. Secondary batteries currently available, however, have a complicated internal structure so that there is a limit in improving the electric capacity of a product which already has a certain capacity. The complex structure thereof is partly responsible for hindering the improvement of reliability of the battery. For example, a current collecting tab or current collecting lead connected to an electrode can inhibit uniform electrode reaction on the electrode surface. In addition, internal short-circuiting which occurs in the case where metal burrs larger than normal are left in the cutting plane of the lead is also a matter for concern.
An electrochemical device comprises an electrode assembly comprising a positive electrode, a negative electrode and a separator. The electrode assembly has two types: stack type and spiral type. A stack type electrode assembly is produced by placing positive and negative electrodes alternately with a separator interposed therebetween. A spiral type electrode assembly is produced by spirally winding elongated positive and negative electrodes with a separator interposed therebetween. In these electrode assemblies, side faces of the positive and negative electrodes are flush with each other. The positive and negative electrodes have a current collector sheet, the whole of which serves as the conductive area, such as a metal foil. In order to acquire electricity from such side faces without short-circuiting, a current collecting tab or current collecting lead is required.
In view of simplifying the internal structure of the electrochemical device, there has been proposed that a positive electrode is projected from one side face of an electrode assembly and a negative electrode is projected from the other side face opposite to the aforesaid side face, whereby electricity is acquired directly from each of the side faces without a current collecting tab or current collecting lead. For example, for a battery having a stack type electrode assembly, a technique in which projected portions of electrode plates of the same polarity are connected using a metal member is proposed (see Japanese Laid-Open Patent Publication No. 2001-126707). As for a battery having a spiral type electrode assembly, a technique in which a projected core member of an electrode of the same polarity is connected to a flat current collector plate is proposed (see Japanese Laid-Open Patent Publication No. 2000-294222).
A conventional current collector sheet for carrying an electrode material mixture is accompanied by the problem of an increased possibility of internal short-circuiting because the entire sheet serves as the conductive area and a portion where an electrode material mixture is not applied inevitably possesses conductivity. Additionally, in the case where a positive electrode is projected from one side face of an electrode assembly and a negative electrode is projected from the other side face opposite to the aforesaid side face, the structure of a current collector will be complicated and therefore it is required to produce electrode assemblies one by one, making the production process of the electrode assembly complicated. In short, a conventional current collector sheet also has the problem that a plurality of electrode assemblies cannot be produced at one time.
Meanwhile, in the development of producing compact secondary batteries, studies have been conducted on a secondary battery comprising a bipolar type electrode which has an advantage of less number of components (see Japanese Laid-Open Patent Publication No. 2000-30746). It is, however, crucial to prevent short-circuiting due to an electrolyte in the secondary battery comprising a bipolar type electrode. Accordingly, there is need for something to be done to its production process.
A bipolar type electrode is mainly used in a stack type electrode assembly although most of secondary batteries employ a spiral type electrode assembly. The reason why a bipolar type electrode is not used in a spiral type electrode assembly is because it makes the structure of the electrode assembly complicated. In order to ensure insulation between electrodes in a spiral type electrode assembly, various measures will be necessary, which will make the structure complicated, so that there is no advantage in using a bipolar type electrode.
As for a common spiral type electrode assembly, on the other hand, its typical production requires four hoops in total: a hoop comprising an elongated positive electrode, a hoop comprising an elongated negative electrode and two hoops comprising a separator. This makes its production process complicated, likely to cause problems such as displacement during spiral winding process.